This invention relates to an operation control system of a rotary displacement type vacuum pump, such as a screw type vacuum pump, scroll type vacuum pump, and more etc., particularly, to a rotary displacement type vacuum pump which is oil-free.
One type of volume control system of a screw compressor is disclosed in, for example, U.S. Pat. No. 4,219,312 and, while it is not generally known to use this type of screw fluid machine as a vacuum pump, studies have shown that a screw fluid machine serving as a compressor can be used as a vacuum pump.
However, to use a screw fluid machine of the prior art as a vacuum pump, a number of problems must be resolved.
More particularly, vacuum pump raises the pressure of gas which is below 10.sup.-2 Torr to the atmospheric pressure level (760 Torr) before discharging same. Thus, it has a high ratio of suction pressure to discharge pressure, with a result that it has a high volume ratio (the ratio of the volume of the working chamber when suction is finished to the volume of the working chamber when discharge is started). On the other hand, when the machine is started, suction pressure is at the atmospheric pressure level. Thus, a starting torque is high because the machine is started when suction pressure is much higher than when a steadystate operation is performed.
Additionaly, high starting torque makes it necessary to use an electric motor of a capacity which is much greater than the capacity required for the steadystate operation of the machine.
Furthermore, the rpm. of the electric motor for driving the machine is generally constant, so that it is difficult to maintain a predetermined level suction pressure or the pressure in a vessel communicating with the suction side of the machine during the steadystate operation.